1. Field of the Invention
This invention relates to a method of epitaxially growing a single or mixed crystal of III-V compound semiconductor such as gallium arsenide (GaAs) and the like on a semiconductor substrate.
2. Description of the Prior Art
Previously, a metalorganic molecular beam epitaxy (MOMBE) has been known as an example of crystal growth methods of a III-V compound semiconductor such as gallium arsenide (GaAs) and the like. In the MOMBE method, group III and V materials each as a molecular beam are supplied on the surface of a semiconductor substrate in a growth chamber held at an ultra-high level of vacuum as in the case of a general molecular beam epitaxy (MBE), but different therefrom in that the MOMBE method uses an organic metal compound as the group III material. Compared with the MBE method, the MOMBE method is advantageous in that such a crystal layer that has reduced surface defect can be obtained at a superior selective growthability. Compared with a metalorganic vapor phase epitaxy (MOVPE), the MOMBE method is also advantageous in that the composition and thickness of a layer to be formed can be highly controlled as well as the uniformity thereof can be made superior, which means that the method offers an extremely effective technique as the crystal growth method of compound semiconductor.
The MOMBE method has been used for producing various semiconductor devices. For example, in the case of growing a single crystal of gallium arsenide (GaAs) semiconductor, alkyl compounds such as trimethyl gallium (TMGa), triethyl gallium (TEGa) and the like, each having an alkyl chemically bonded to the gallium atoms, are used as a gallium source material. In the case of growing a single crystal of aluminum arsenide (AlAs), alkyl compounds such as trimethyl aluminum (TMAl), triethyl aluminum (TEAl) and the like, each having an alkyl chemically bonded to the aluminum atoms, are used as an aluminum source material. In any case, as an arsenic source material, metallic arsenic (As4) or arsenic hydrides such as arsine (AsH.sub.a) and the like are used. In making a heterojunction bipolar transistor (HBT), as the group III source material, for example, TEGa, TMGa or TEAl is used and as the group V source material, for example, AsH.sub.a is used.
On the other hand, as the group III source material, such an organic metal compound that has a methyl chemically bonded to the metal atoms as TMGa is used, the carbon contained in the methyl is taken during crystal growth into an epitaxially grown layer, thus being inclined that the epitaxially grown layer is susceptible to exhibit a p-type electroconductivity. For this, investigation results were reported, for example, by N. Furuhata and other researchers in "Journal of Crystal Growth", Vol.102, p.814, 1990, in which TEGa having an ethyl is used as the group III source material and As.sub.4 was used as the group V source material, and a single crystal of GaAs was epitaxially grown. This report reveals that under such growth conditions that a temperature of the substrate is 500.degree. C., flow rate of TEGa is 0.8 cc/min and pressure of As.sub.4 is 1.5.times.10.sup.-4 Torr, such a high purity GaAs single crystal can be obtained that has a p-type electroconductivity, a carrier concentration of 1.5.times.10.sup.14 cm.sup.-3 and a carrier mobility at room temperature of 400 cm.sup.2 /V.s.
A single crystal of III-V compound semiconductor with a low dopant concentration or with a high purity, as the group III source material, an organic metal compound having an alkyl (for example, ethyl) which is easy to be desorbed from the group III element atoms compared with the methyl is generally used in general. When such an organic metal compound that has an alkyl as shown above is used, if an n-type impurity such as silicon (Si) and the like is doped during growth, an n-type single crystal can be obtained which is low in compensation ratio.
With the conventional MOMBE method, in case that an organic metal compound such as TEGa as the group III source material is used to grow a single crystal of high purity (low impurity concentration) or n-type GaAs semiconductor, there arises such a problem that a good selective growthability of the single crystal cannot be obtained unless the growth temperature, that is, temperature of the substrate exceeds 600.degree. C. Namely, when organic metal compound molecules as TEGa and As molecules are supplied from the upper side of a mask (for example, made of a silicon dioxide) formed on the surface of a GaAs substrate thereby to epitaxially grow a single crystal of GaAs semiconductor only on the unmasked area of the surface of the substrate, if only one kind of organic metal compound is supplied as the group III source material as in the conventional MOMBE method, the single crystal of GaAs semiconductor can be grown on the unmasked surface of the substrate at a temperature not exceeding 600.degree. C., however, polycrystalline GaAs semiconductor is deposited simultaneously on the mask, thus being unobtainable a satisfactory selective growthability. Such a phenomenon is outstanding when an organic metal compound having an ethyl as TEGa is used as the group III source material.
Detailed and concrete explanations will be made below. When a high purity GaAs single crystal is grown using TEGa only as the group III source material using the MOMBE method, the temperature of a substrate is preferably to be set at about 500.degree. C. This is based on the fact that if it exceeds 500.degree. C., a p-type GaAs crystal with a high impurity concentration is grown thereby being unobtainable a high purity GaAs crystal. In addition, even when an n-type GaAs crystal having a low compensation ratio is to be grown by this method, it is preferable to set the temperature of the substrate at about 500.degree. C. This is based on the fact that at a higher temperature than 500.degree. C., a high purity GaAs crystal cannot be grown on the substrate as already shown above. As a result, even if the single crystal of GaAs is grown while doping an n-type impurity at a high concentration, only the single crystal of n-type GaAs semiconductor extremely high in compensation ratio is grown.
Then, if using TEGa only as the group III source material, a high purity GaAs single crystal or low compensation ratio n-type GaAs single crystal is grown on the surface of a GaAs substrate with a mask formed thereon using the MOMBE method at a substrate temperature of about 500.degree. C., it can be grown on the unmasked area of the surface of the substrate and at same time, polycrystalline GaAs is deposited on the masked area thereof. This is schematically illustrated in FIG. 1.
In FIG. 1, on the surface of a GaAs substrate 1 is formed a mask 2 made of a silicon dioxide (SiO.sub.2) film. On the exposed surface of the GaAs substrate 1 where the mask 2 is not provided to form an opening, a single-crystalline GaAs layer 3 is grown and a thick polycrystalline GaAs layer 4 is grown on the mask 2.
As explained above, with the conventional MOMBE method, if the temperature of a substrate is set at a low level of about 500.degree. C., it is impossible to selectively grow a high purity GaAs single crystal or n-type GaAs single crystal having a low compensation ratio on the substrate and as a result, with the conventional MOMBE method, devices such as, for example, a heterojunction field effect transistor having a low source resistance cannot be produced by selectively growing an n-type single-crystalline GaAs layer having a high impurity concentration only at an ohmic electrode portion.
Thus, an object of this invention is to provide a crystal growth method of a III-V compound semiconductor in which a single crystal of III-V compound semiconductor can be satisfactorily selectively grown on a substrate by setting a temperature of the substrate at a lower level (for example, at about 500.degree. C.) than would be set conventionally.